The present invention relates generally to computerized tomography (CT) systems. More particularly, the present invention relates to an apparatus and method for receiving high data rate communication in a CT system.
CT systems are used to obtain non-invasive sectional images of test objects, particularly internal images of human tissue for medical analysis and treatment. Current CT systems position the test object, such as a patient, on a table within a central aperture of a rotating frame, or gantry, which is supported by a stationary frame. The gantry includes an x-ray source and a detector array positioned on opposite sides of the aperture, within an x-y plane of a Cartesian coordinate system (generally referred to as the xe2x80x9cimaging planexe2x80x9d), such that both rotate with the gantry around the test object being imaged. At each of several angular positions along the rotational path of the gantry (also referred to as xe2x80x9cprojectionsxe2x80x9d), the x-ray source emits a fan-shaped collimated beam which passes through the imaging slice of the test object, is attenuated by the test object, and is received by the detector array. Each detector element in the detector array produces a separate electrical signal indicative of the attenuated x-ray beam intensity, the beam projected from the x-ray source to the particular detector element, incident at its sensor surface. The electrical signals from all the detector elements are collated by circuitry within the rotating frame to produce a projection data set at each gantry angle or projection. Each projection data set is referred to as a xe2x80x9cviewxe2x80x9d, and a xe2x80x9cscanxe2x80x9d is a set of such views from the different gantry angles during one revolution of the x-ray source and detector array. The scan is then processed by a computer in the stationary frame to reconstruct the projection data sets into a CT image of the slice or cross-section of the test object.
To transfer the projection data sets from the rotating frame to the stationary frame for image reconstruction, various communication links such as an umbilical cable, an optical data link, a slipring with a contacting brush, and a slipring with a contactless coupler are currently available. Newer CT systems typically utilize a slipring disposed on the rotating frame with a contactless coupler, having a certain air gap with respect to the slipring, disposed on the stationary frame as the communication link between the rotating and stationary frames. The slipring comprises a broken circle of wire or transmission line encircling the aperture of the rotating frame such that each half of the broken circle of wire forms an arc of exactly the same length. Data signals, e.g., the projection data sets, are encoded and transmitted from the first ends of the two wires to the second ends of the two wires at the opposite side of the broken circle such that both data signals arrive at the second ends, generally referred to as the termination gap, at the same time. The contactless coupler disposed on the stationary frame lies close to the slipring and captures the transmitted encoded data signals via electromagnetic coupling. Because each projection data set is transmitted as they are acquired (after encoding), i.e. while the rotating frame is still rotating to acquire the next projection data set for the next gantry angle, propagation of the data signals along the wires of the slipring and electromagnetic coupling from the slipring to the contactless coupler occurs while the rotating frame and thus the slipring are in rotation.
Unfortunately, although the slipring and contactless coupler provides many advantages over other types of communication links such as higher data rate transfer, shorter image acquisition time, increased patient comfort, and less mechanical stress and wear, it also suffers from problems associated with controlling the data signal strength present at the contactless coupler. Data signals received by the contactless coupler can have a power variation of up to 15 to 20 dB around its center operating power level. It has been found that numerous sources contribute to the signal strength variability, including, but not limited to: (1) axial and/or radial rotation runouts of the rotating assembly; (2) axial and/or radial misalignment of the contactless coupler; (3) variability of the printed circuit board within the channel groove; (4) nominal air gap losses; (5) accuracy of the dimensions of the various components of the rotating assembly such as the channel groove; (6) skin effect and dielectric losses in ring circuit board traces; and (7) amplifier gain variability. Moreover, although attempts have been made to correct signal variation resulting from any one contributing source, it is difficult, time consuming, and costly to correct the variations caused by all of these contributing sources given the interaction between these sources in CT systems.
Thus, there is a need for an apparatus and method capable of accommodating wide variations in the data signal strength received by a contactless coupler in a CT system. Moreover, there is a need for such an apparatus and method to be efficient, cost effective, flexible to the range of signal variations, and to enhance, or at least not counteract, the advantageous capabilities of a communication link comprised of a slipring with a contactless coupler in a CT system.
One exemplary embodiment relates to an apparatus for demodulating and digitizing a modulated data signal possibly having a wide range of power levels. The apparatus includes a variable attenuation circuit configured to attenuate an amplitude of the modulated data signal in response to a feedback signal to generate an amplitude stabilized signal, and a conversion circuit coupled to the variable attenuation circuit and configured to remove a modulation component from the amplitude stabilized signal to generate a digitized signal. The apparatus further includes a circuit coupled to the conversion circuit and configured to generate a reconstructed digital data signal and an amplitude signal from the digitized signal, and an attenuation controller coupled to the circuit and the variable attenuation circuit. The reconstructed digital data signal is representative of the modulated data signal in a pre-modulated state. The amplitude signal is representative of an amplitude of the digitized signal. The attenuation controller is configured to generate the feedback control signal from the amplitude signal.
Another exemplary embodiment relates to an apparatus for demodulating and digitizing a modulated data signal possibly having a wide range of power levels. The apparatus includes means for variably attenuating the modulated data signal in response to a feedback signal to generate an amplitude stabilized signal, and means for digitizing the amplitude stabilized signal to generate a digitized signal. The apparatus further includes means for filtering the digitized signal to generate a reconstructed digital data signal representative of the modulated data signal in a pre-modulated state, and means for generating the feedback signal in response to the digitized signal. The feedback signal is representative of an amplitude of the digitized signal.
Still another exemplary embodiment relates to a method for demodulating and digitizing a modulated data signal possibly having a wide range of power levels. The method includes transmitting the modulated data signal from a first location, and receiving the modulated data signal at a second location. The first and second locations are remote from each other. The method further includes attenuating an amplitude of the modulated data signal in response to a previous modulated data signal via a feedback loop, and converting the modulated data signal into a reconstructed digital data signal. The reconstructed digital data signal is representative of the modulated data signal in a pre-modulated state. The attenuating and converting steps are carried out at the second location. The modulated data signal comprises a signal having a power level range of up to 50 dBm around a center operating power level.